1. Field of the Invention
The present invention relates generally to an optical illumination system for a projection system, and more particularly to an optical device with a function of homogenizing and color separation and an optical illumination system for a projector whose size can be reduced by use of the optical device.
2. Description of the Prior Art
Recently, as a kind of a flat display, which is thin in its thickness and can realize a large screen, substituted for a cathode ray tube display, which is limited in its screen size and large in its system size, projectors for projecting pictures of a small-sized screen on a large-sized screen with a magnification are coming into rapid and wide use.
The projectors, which are display devices for realizing pictures on small-sized screen, can employ a cathode ray tube, an LCD (Liquid Crystal Display) or a DMD (Digital Micromirror Device), but use mainly the LCD or the DMD according to a trend of down-sizing in thickness.
The LCD realizes pictures by changing an alignment state of liquid crystal molecules depending upon electrical variations from the external, and controlling an amount of light transmission based on the changed alignment state of liquid crystal molecules. The DMD realizes pictures by changing inclination angles of micromirrors between +10xc2x0 and xe2x88x9210xc2x0 depending upon electrical variations from the external, such that a reflection angle of light has two modes.
Such projectors are currently developing with the most important point put on high brightness, miniaturization and lightweight.
More particularly, the projectors are being improved to have a vivid screen even under bright surroundings by employing a lamp used as a source of light with a small magnitude of light emission, fly eye lenses for homogenizing an amount of light, polarization conversion devices for converting light emitted from the source of light into linear polarized light, etc., such that an efficiency of light is increased.
In addition, for the miniaturization and lightweight, the projectors are developing from a three-plate system for realizing colors using three display elements to a single-plate system for realizing colors using one display element.
The projectors employing the single-plate system using one display element use a method of color filters for realizing colors, a method of sequentially providing three primary colors for the display element, a method of separating and scrolling three primary colors, etc.
Among these methods, an optical illumination system using three rotating prisms for changing a traveling direction of colored light and scrolling the colored light separated from dichroic mirrors for color separation can be representative of the method of separating and scrolling three-primary colors.
FIG. 1 is a view showing a structure of a conventional optical illumination system of a projector employing a single-plate system using three rotating prisms.
Referring to FIG. 1, the optical illumination system includes first and second fly eye lenses 4 and 6, a polarizing beam split (referred to as PBS hereinafter) array 8, first to fourth dichroic mirrors 12, 24, 32 and 44 for color separation, first and second total reflection mirrors 16 and 40 for totally reflecting incidence light, first to third rotating prisms 18, 26 and 28 for changing an optical path depending on their rotation angles, first to seventh condensing lenses 10, 14, 30, 36, 38 and 46 for condensing light, first and second relay lenses 34 and 42 for relaying an image formation point, and a PBS prism 50, all of which are arranged on an optical path between a source of light 2 and a display device 52.
Now, an operation of the optical illumination system of the projector as shown in FIG. 1 will be described.
The first and second fly eye lenses 4 and 6 make light distribution uniform by dividing white light from the source of light 2 by the unit of lens cell and outputting the divided light to the PBS array 8.
The PBS array 8 separates the incidence light into linear polarized light having one of optical axes, i.e., P polarized light and S polarized light. Here, the S polarized light is outputted as it is, and the P polarized light is converted and outputted into S polarized light by a xc2xd wavelength plate (not shown) partially attached on a back side of the PBS array 8, such that a state of polarization becomes uniform. The first condensing lens 10 condenses the light outputted from the PBS array 8 into the first dichroic mirror 12.
The dichroic mirror 12 is made of a blue reflection coating for reflecting blue light, and green and red transmission coatings for transmitting green and red light, respectively. The first total reflection mirror 16 totally reflects blue light, which is reflected by the first dichroic mirror 12 and inputted through the second condensing lens 14, into the first rotating prism 18.
The second dichroic mirror 24 made of a green reflection coating and a red transmission coating reflects green light into the second rotating prism 26 and transmits red light into the third rotating prism 28, both of green and red light being incidence light transmitted by the first dichroic mirror 24 and inputted through the third condensing lens 20.
The first to third rotating prisms 18, 26 and 28 change traveling directions of blue, green and red light depending on their rotation angles, respectively. More particularly, The first to third rotating prisms 18, 26 and 28 change image formation positions of blue, green and red light at which images are formed on the display device 52 depending on their rotation angles, respectively, and scroll the image formation positions of the three color light sequentially, while they are rotating independently.
The blue light transmitted through the first rotation prism 18 is inputted to the fourth dichroic mirror 44 via the fourth condensing lens 30, the third dichroic mirror 32, and the first relay lens 34. The green light transmitted through the second rotation prism 26 is inputted to the fourth dichroic mirror 44 via the fifth condensing lens 36, the third dichroic mirror 32, and the first relay lens 34. The red light transmitted through the third rotation prism 28 is inputted to the fourth dichroic mirror 44 via the sixth condensing lens 38, the second total reflection mirror 40, and the second relay lens 42.
The third dichroic mirror 32 is made of a red reflection coating for totally reflect the red light from the second rotating prism 26 and a blue transmission coating for transmitting the blue light from the first rotating prism 18. The fourth dichroic mirror 44 reflects the incidence blue light and green light and transmits the incidence red light.
Each of the red, green and blue light transmitted or reflected by the first to fourth dichroic mirrors 12, 24, 32 and 44 has a S polarization component and is inputted to the PBS prism 50 via the seventh condensing lens 46 and a polarization plate 48.
The S polarized light inputted from the polarization plate 48 to the PBS prism 50 is reflected at a polarized light split surface 50A into the display device 52. In this case, based on different initially set rotation angles of the first to third rotating prism 18, 26 and 28, the red, green and blue light form images on different portions of the display device 52. The different image formation positions are scrolled in a specific direction when the first to third rotating prism 18, 26 and 28 are driven. The display device 52 scrolls red, green and blue signals in accordance with the red, green and blue light inputted while the different image formation positions are speedily scrolled.
Accordingly, each of the three color signals is implemented in an according pixel of the display device 52 and the implemented three color signals are integrated with time for displaying a color picture. In case that the display device 52 is a reflection-typed liquid crystal display device, the S polarized light inputted from the PBS prism 50 is converted into P polarized light depending on video signals for implementing a color picture. The color picture with the P polarized light component implemented in the display device 52 is projected on a screen with a magnification via the PBS prism 50 and a projection lens (not shown).
However, the projectors employing the single-plate system using the three rotating prisms as described above have a problem that it is difficult to accomplish a synchronization in time among the three rotating prisms.
More particularly, although the synchronization in time among the three rotating prisms is initially accomplished, a difference in the synchronization in time among the three rotating prisms is increasingly generated by a variation among drivers of the rotating prisms as a period of time elapses after the optical illumination system is organized. Thus, when the synchronization in time among the three rotating prisms becomes different, it is impossible to implement a desired color on the screen.
Also, in addition to the three rotating prisms and a plurality of dichroic mirrors for color separation, since the conventional optical illumination system for the projector employing the single-plate system further requires motors for driving the three rotating prisms, it has problems that the optical illumination system becomes complicated in its structure and large in its volume due to a relatively more space occupied by them, resulting in a difficulty of miniaturization and down-sizing in thickness of the system.
Accordingly, the present invention has been made keeping in mind the above problems occurring in the prior art, and an object of the present invention is to provide an optical device with a function of homogenizing and color separation and an optical illumination system for a projector whose size can be reduced by use of only the optical device and one prism.
The optical illumination system for the projector employing a single-plate system according to the present invention has an advantage in that homogenization of optical distribution and improvement of optical efficiency can be accomplished and the volume of the optical illumination system can be reduced at its maximum, compared to the conventional optical illumination system using the fly eye lenses and the three rotating prisms, by use of only the optical device with the function of homogenizing and color separation and one prism.
In addition, since three colors light is scrolled by use of only one prism, deterioration of picture quality due to a difference in the synchronization in time among the three rotating prisms can be prevented, unlike the conventional optical illumination system using the three rotating prisms.
In the end, the optical illumination system for the projector employing a single-plate system according to the present invention has remarkable advantages in that the miniaturization and lightweight of the projector can be accomplished and picture quality can be improved.